An α-olefin is an industrially important raw material monomer that is produced by the oligomerization of ethylene using a metal catalyst. However, the oligomerization of ethylene usually gives α-olefin mixtures according to Shultz-Flory distribution. Therefore, the development of a catalyst system capable of selectively producing α-olefin is very important industrially.
For example, PATENT DOCUMENT 1 has reported that a half-metallocene titanium complex represented by the formula (Cp-B(R)nAr)TiR13 works as a catalytic component for selective trimerization of ethylene in the presence of an activating co-catalytic component.
Among these catalysts for selective ethylene trimerization, a half-metallocene titanium complex comprising cyclopentadiene bonded to a substituted aryl group via a carbon atom, such as [1-(1-methyl-1-(3,5-dimethylphenyl)ethyl)-3-trimethylsilylcyclopentadienyl]titanium trichloride, has been reported to work as an efficient ethylene trimerization catalyst under condition of 30° C. with MAO (methylaluminoxane) as an activating co-catalytic component (see e.g., NON-PATENT DOCUMENT 1). On the other hand, [dimethylphenylsilylcyclopentadienyl]titanium trichloride comprising cyclopentadiene bonded to a phenyl group via a silicon atom has been reported to have low catalytic activity in ethylene trimerization reaction under the same condition as above and to produce a large amount of polyethylene as a by-product (see NON-PATENT DOCUMENT 1).
Moreover, it has been reported that a catalyst system for ethylene trimerization using a similar half-metallocene titanium complex comprising cyclopentadiene bonded to a substituted aryl group via a carbon atom exhibits much lower catalytic activity in 1-hexene production and 1-hexene production selectivity under high temperature conditions of 80° C. than at 30° C. (see NON-PATENT DOCUMENT 2).
An ethylenic copolymer having a main chain with ethylene units and an alkyl side chain (e.g., an ethyl or butyl branch), for example, linear low-density polyethylene, has conventionally been produced by copolymerizing ethylene and α-olefin (e.g., 1-butene or 1-hexene) in the presence of an olefin polymerization catalyst.
However, the conventional process for producing an ethylenic copolymer requires using ethylene and expensive α-olefin as raw material monomers and was thus a less-than-sufficient process economically. Thus, a process for producing an ethylenic polymer having an alkyl side chain has been studied in recent years, which utilizes a tandem polymerization process in which only ethylene is used as the raw material monomer and an ethylene oligomerization catalyst and an olefin copolymerization catalyst are used in one reactor.
For example, a process for producing an ethylenic polymer having a butyl branch has been proposed, which comprises polymerizing ethylene in the presence of an olefin polymerization catalyst obtainable by bringing dimethylsilylene(tert-butylamido)(tetramethylcyclopentadienyl)titanium dichloride as an olefin copolymerization catalyst, [1-(1-methyl-1-phenylethyl)-cyclopentadienyl]titanium trichloride as an ethylene trimerization catalyst, and MMAO as an activating co-catalytic component into contact with each other (see NON-PATENT DOCUMENTS 3 and 4). It has been reported therein that an ethylenic polymer having a wide range of melting points and crystallinity is obtained by changing the mixing ratio of the olefin copolymerization catalyst and the ethylene trimerization catalyst and that the ethylenic polymer obtained at 45 to 50° C. however has a much lower 1-hexene content than that obtained at 25 to 30° C. (see NON-PATENT DOCUMENT 3). It has also been reported that particularly at 70° C., the incorporation of 1-hexene is not observed and an ethylene copolymer having a melting point as very high as 133.6° C. is obtained (see NON-PATENT DOCUMENT 4).
NON-PATENT DOCUMENT 4 discloses that an ethylenic polymer having a low melting point is obtained by polymerizing ethylene in the presence of an olefin polymerization catalyst obtainable by bringing rac-dimethylsilylenebis(2-methylbenz[e]indenyl)zirconium dichloride as an olefin copolymerization catalyst, [1-(1-methyl-1-phenylethyl)-cyclopentadienyl]titanium trichloride as an ethylene trimerization catalyst, and MMAO as an activating co-catalytic component into contact with each other. However, no case has reported polymerization at a temperature exceeding 25° C.